Dual thin film precision resistance trimming

ABSTRACT

A trimmable resistor for use in an integrated circuit is trimmed using a heater. The heater is selectively coupled to a voltage source. The application of voltage to the heater causes the heater temperature to increase and produce heat. The heat permeates through a thermal separator to the trimmable resistor. The resistance of the trimmable resistor is permanently increased or decreased when the temperature of the resistor is increased to a value within a particular range of temperatures.

BACKGROUND

1. Technical Field

This description generally relates to the field of resistance trimming.

2. Background

Traditional methods of trimming resistors which are embedded withinintegrated circuits are limited to pre-assembly stages of manufacturingor require forfeiture of significant silicon real estate to obtainprecision trims. One method includes using a laser to cut away portionsof the resistor to alter its resistance. This method requires access tothe fabrication layer in which the resistor is deposited. Additionally,this method cannot be accomplished during the application of the devicecontaining the resistor. Another method includes electrical fusing.Electrical fusing, however, is limited to a binary correction of theresistance. In the alternative, electrical fusing may utilize large fusebank structures to produce more precise changes in resistance. Thetradeoff for more precise resistance trimming then becomes forfeiture ofvaluable silicon real estate. In summary, traditional resistor trimmingmethods require visible access to the resistor of interest, or themethods require forfeiture of silicon real estate to effectuate preciseresults.

BRIEF SUMMARY

The following disclosure relates to a method of trimming a resistorusing a heater disposed in close proximity to the resistor in anintegrated circuit. By applying a voltage to the heater, the temperatureof the heater will increase. The generated heat permeates through athermal conductor to the resistor, raising the temperature of theresistor. The composition of the resistor material is selected to be ofa type that the resistance value of the resistor permanently changesbased on the temperature the resistor is exposed to. Separating theheater from the resistor by one or more thermal separators and applyinga voltage to the heater results in changing the resistance of theresistor. The resistance can be set at a selected value over a widerange of possible resistances.

In one embodiment, the resistor is connected to an amplifier in afeedback configuration that will affect the gain. The gain of anoperational amplifier (“op amp”) circuit can be controlled by the valueof a feedback resistor. If a precise gain is needed by the circuit, thefeedback resistor can be precisely tuned to achieve a desired op ampperformance.

In another embodiment, the resistor is trimmed and used in a currentsense configuration. In a current sense configuration, a resistor may beplaced across the input terminals of an op amp. The op amp is used tosense and amplify the voltage drop across the resistor. Knowing theexact resistance of the resistor and the gain of the op amp will allowone to determine the amount of current flowing through the resistor forevaluating the parameter of interest. The value of the resistor can bemore precisely set than just using standard process manufacturingtechniques. The value of the resistance of the resistor will affect therange of currents detectable by the current sense configuration. Forsome sense circuits, a very low resistor may be preferred, while inother types of sense circuitry a higher resistance will provide moreaccurate sensing and better circuit performance. Thus, the ability toadjust the resistor may improve the effectiveness of a current sensecircuit.

According to the embodiments provided herein, the value of the resistorcan be modified after the integrated circuit is completed as well as atnumerous different stages in the process. After the circuit has beencompletely formed, the circuit performance can be tested to determinethe characteristics and parameters of the individual circuit componentsand transistor operation. The desired resistance value can be determinedbased on these tests.

When a circuit is designed, it is designed towards a target performanceand specification. When the circuit is actually construed in silicon,the actual performance of the various components, such as transistors,capacitors and resistors, will be slightly different from the designedtarget value. One of the most difficult components to build to the exactdesign specifications is a resistor in silicon or polysilicon or a HIPOresistor. Accordingly, the circuit can be tested after it is constructedto determine the actual performance of the components. It can be testedat several locations, for example, the sheet resistance of the resistorcan be tested, or the performance of the circuit, or the gain of theamplifier or some other results of the circuit can be tested todetermine the actual characteristics of the circuit as actually formed.After this step, the resistance value needed to achieve a desiredperformance can be calculated. Then, the value of the resistance can bemodified to be closer to the target value so that the circuit performscloser to the target specification.

As explained in more detail herein, after one or more tests areperformed, the resistance is modified to have a more exact resistancevalue to provide the preferred performance characteristics for theparticular circuit. The proper voltage is applied to the heater to causethe heater to reach desired temperature for heating of the resistor. Theresistor is heated to a selected temperature which will cause it toreach a permanent resistance value. The heater is then turned off, andthe resistance value of the resistor will have been placed at thedesired value in order to achieve the preferred circuit performance. Thevalue of the resistance has therefore been trimmed to a precise desiredvalue.

This trimming of the resistor can occur at many different stages in thecircuit process. It can occur at the wafer test stage in which the waferis tested for circuit performance and operation, as is often done totest op amp performance or circuit feedback characteristics. Once the opamp characteristics are known from the test during the wafer testing,the preferred value of the resistance can be calculated and thereafterthe appropriate voltage applied to the heater to cause the resistor tobe trimmed to the selected resistance.

Alternatively, the resistive trim stage can occur at the individual diestage. After the wafer has been diced and individual dies are separated,in some embodiments, the die may be tested prior to packaging. Eachindividual die may be tested to determine the actual circuit parametersand then a desired resistance value. At this stage, the appropriateprobes can be applied in order to heat the resistor to the desired valueand therefore trim the resistance.

Alternatively, the resistor trimming can occur after the die ispackaged. In this embodiment, the die is completely packaged and readyfor use. Just prior to shipment, the manufacturer may place the die in atest socket and test various components of the circuit operation. If oneor more resistors have a value which needs modification, then a voltagecan be applied to the appropriate heater and the resistance can bemodified to have the selected value in order to provide the desiredcircuit performance.

Alternatively, the purchaser of the chip may also trim the resistor justprior to using it in an end circuit. The user, upon purchase of the chipin its complete package form, may perform various tests on theintegrated circuit package according to their desired end use of thecompleted circuit. The user can determine what resistance value ispreferred and, by applying appropriate voltage to the terminals of thepackage, heat the resistor to the selected value and thus change theresistor to have a value which provides the preferred circuitperformance.

In summary, the value of the resistance can be trimmed at various stagesin the product cycle, including at the initial manufacturing stage, atthe wafer probe test stage, at the die test stage, at the packaged teststage, or, in some embodiments, even by the user after the chip has beenpurchased on the commercial market. This provides a wide range ofalternative stages over which the value of the resistor can be trimmedwell beyond what was possible in the prior art.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. Further, some figures are inschematic form and the particular shapes of some of the elements asdrawn are not intended to convey information regarding the actual shapeof the particular elements and have been selected for ease ofrecognition in the drawings.

FIG. 1 is block diagram illustrating resistor trimming, in accordancewith an embodiment of the invention.

FIGS. 2 through 11 are cross-sectional views illustrating the pluralityof processing steps that may be used in manufacturing the layersassociated with the heater and trimmable resistor of FIG. 1, inaccordance with an embodiment of the invention.

FIG. 12 is a partial-layer top view of the heater and trimmableresistor, in accordance with an embodiment of the invention.

FIG. 13 is a partial-layer top view of the heater and trimmableresistor, in accordance with another embodiment of the invention.

FIG. 14 is a layout view of a pair of trimmable resistors and resistorheaters, in accordance with an embodiment of the invention.

FIG. 15 is a cross-sectional view of various layers of the heater, thetrimmable resistor, and a transistor, in accordance with an embodimentof the invention.

FIG. 16 is a circuit diagram illustrating an amplifier configurationusing the trimmable resistor, in accordance with an embodiment of theinvention.

FIGS. 17A and 17B are circuit diagrams illustrating an integratedcircuit using the trimmable resistor, in accordance with an embodimentof the invention.

FIGS. 18A-18C are flowcharts illustrating various methods of trimming atrimmable resistor, in accordance with embodiments of the invention.

FIG. 19 is a chart illustrating how the sheet resistance of a trimmableresistor may permanently change with temperature, in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various disclosedembodiments. However, one skilled in the relevant art will recognizethat embodiments may be practiced without one or more of these specificdetails, or with other methods, components, materials, etc. In otherinstances, well-known structures and methods associated with integratedcircuits and semiconductor manufacturing/packaging processes have notbeen shown or described in detail to avoid unnecessarily obscuringdescriptions of the embodiments.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, suchas, “comprises” and “comprising” are to be construed in an open,inclusive sense, that is, as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contextclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contextclearly dictates otherwise.

FIG. 1 shows one embodiment of a trimming system 100 for trimming orchanging the sheet resistance of a resistor. The embodiment includes anintegrated circuit 101, an input signal 102, a switch 104, a heater 106,and a resistor 108. Additionally, trimming system 100 has a voltagesource V_(h), which provides a current I_(h) to flow through heater 106.The resistor 108 is connected to an integrated circuit, shown here onlyin block diagram form.

Switch 104 is connected to heater 106 to selectively supply a voltageV_(h). The switch opens and shuts in response to input signal 102. Inone embodiment input signal 102 is a single pulse. In anotherembodiment, input signal 102 is a series of pulses. Switch 104 can beimplemented using a variety of devices, including a transistor, eitherbipolar or MOS, or a triac, thyristor, or other switchable supply of aheating current I_(h).

Heater 106 receives a voltage potential V_(h) from switch 104. The valueof the voltage potential V_(h) is selected to achieve a desired currentI_(h) through the heater 106 based on the type of switch 104 used. Insome instances, in which the switch 104 is an on/off switch thatprovides the full value V_(h) to the heater 106, the value of V_(h) isselected to provide a desired current I_(h) for heating the resister.This can be either a single pulse of a given voltage or, preferably, asa series of small pulses so that each incremental pulse providesincremental heating of the heater 106. A pulse wave modulation signal(PWM) can be applied via signal 102 to the switch 104. Alternatively,the voltage V_(h) can be a known rail voltage, such as Vcc, Vpp, orother voltage rail available on the integrated circuit. In this case,the signal 102 can be applied as a pulse width modulation signal toprovide short ws of current flow I_(h) to the heater 106. Alternatively,the signal 102 can be an analog-driven signal to provide a selectedcurrent flow through the heater 106. In such instances of an analogdrive signal 102, a bipolar transistor is preferred for the switch 104.In other instances, a triac, thyristor, or MOS transistor may be usedfor the switch 104 in order to achieve a desired temperature in theheater 106.

Current I selectively flows through heater 106 to cause power todissipate in the device. The temperature of the heater is defined by theJoule effect of power dissipation. This effect is mathematicallyrepresented as:

Q=R _(heater)×(I _(heater))² ×t,

where

Q=the heat generated by a constant flow of current;

R_(heater)=the resistance of the heater;

I_(heater)=the current flowing through the resistor; and

t=time, the duration current is flowing through the resistor.

Power dissipating in heater 106 results in the temperature of heater 106increasing. As will be discussed in association with FIG. 7, thecomposition of heater 106 will affect its resistance and thereforethermal response to current I passing through it. The heat generated byheater 106 permeates to resistor 108.

Resistor 108 is depicted in trimming system 100 as being in closeproximity to heater 106. The proximity of resistor 108 to heater 106will, in part, determine the amount of heat transferred between thedevices, i.e., the thermal coupling. Whereas the resistance of manyresistors temporarily varies with temperature, resistor 108 is of a typethat will permanently change resistance upon exposure to a temperaturewithin a range of temperatures. It can be of a type in which theresistance increases when subjected to heat, or the resistance decreaseswhen subjected to heat.

A trimmable resistor, such as resistor 108, adds tremendous value tointegrated circuit manufacturing in the way of precision while requiringvery little additional hardware overhead. A voltage source is alreadyavailable on all integrated circuits and in any wafer probe test. Inaddition, switches 104 are also available in standard integratedcircuits 101. The additional hardware of a heater 106 and heater controlcircuit will be provided, however, such heaters and control circuits caneasily be formed in standard integrated circuit processes and will onlytake up a small amount of space. A trimmable resistor such as isdisclosed herein allows a large spectrum of trimming capabilities. Inone embodiment, resistor 108 is part of a low-pass or high-pass RCfilter where resistor 108 is inversely proportional to the cut-offfrequency. In another embodiment, resistor 108 is the gain controllingfeedback resistor of an amplifier circuit. In yet another embodiment,resistor 108 is a current sense resistor which may be connected acrossthe inputs of a current sense amplifier. Since integrated circuits whichproduce results to within tight tolerances of the customer specificationare of more value and if far outside of a customer specification oftenhave to be discarded or sold undervalue, the ability to produce higherprecision circuits becomes immensely valuable in a circuit manufacturingenvironment.

FIG. 2 shows a cross-sectional view of layers representing theintegrated circuit 101 used in the fabrication of portions of trimmingsystem 100. Trimming system layers 100 includes a silicon substrate 202,an insulator 204, a metal layer 206, and a layer 208.

Silicon substrate 202 may be a substrate of monosilicon. It may be alayer of polysilicon grown above a substrate or another process layer onwhich insulator 204 may be disposed. In one embodiment, silicon layer202 contains active devices, while in another, it is on top of activedevices, consequentially increasing the integration density of anintegrated circuit upon which various layers may be disposed. Thesubstrate 202 will have a number of other circuit components formedtherein according to techniques well known in the art. For example,substrate 202 will include some or all of the components of the circuitof FIGS. 1, 16 and 17, shown elsewhere herein. The formation oftransistors and interconnections to form specific circuits in asemiconductor substrate to make an integrated circuit are well known andtherefore not described in detail herein. Any acceptable techniques maybe used which are compatible with the processes and layers in theresistive heating circuit described herein.

Insulator 204 prevents various sections of metal layer 206 from havingundesirable low-impedance connections. Insulator 204 may be one of manydielectrics known by one of skill in the art, including silicon dioxide(SiO₂), phospho-silicate glass (PSG), and boro-phospho-silicate glass(BPSG).

Layer 208 is disposed above insulator 204. Layer 208 acts as a layerthat is an etchable foundation for a successive layer. Layer 208 isideally an inefficient thermal conductor. As such, it protectselectrical structures disposed below from heat dissipated in layersabove, so as not to substantially impact electrical structures such asmetal layers. In one embodiment, layer 208 is made of TEOS with athermal conductivity on the order of 1.1 W/m/K. Layer 208 is etched toact as a socket for the resistor 108, as explained later herein. Inanother embodiment, layer 208 is implemented with an air void. In eitherembodiment, layer 208 is disposed in preparation for the subsequentresistive layer.

FIG. 3 shows the use of a photoresist 210, which is a temporary layer,within trimming system 100 to create via 211 b and via 211 c in layer208 and insulator 204. The layer 206 has electrical isolated metalstructures 206 a, 206 b, 206 c, 206 d, etc. The openings extend to andexpose metal layer 206 b and 206 c. By exposing metal layer 206 b and206 c, the next conductive layer may electrically couple to metal layer206 b and 206 c.

FIG. 4 shows that a resistor 108 has been deposited, the photoresistlayer 212 is patterned and etched to obtain the desired dimensions ofthe resistor 108. Resistor 108 corresponds to trimmable resistor 108 ofFIG. 1. Resistor 108 is connected to metal layer 206 b and 206 c.Resistor 108 has an initial resistance that is determined by itscomposition, length, width, and depth. In one embodiment, resistor 108is a thin film resistor (“TFR”). In one embodiment, resistor 108 is aTFR with an initial sheet resistance of 1 k ohm/sq. Resistor 108 is alsocomposed of material that will permanently change after exposure to arange of temperatures. In one embodiment, resistor 108 comprises CrSi, amaterial with a low temperature coefficient of resistance. In oneembodiment, the range of temperatures which will cause it to changeresistance is 450° C. to 850° C.

As to avoid damage to resistor 108 and overheat proximate electricalstructures, the duration of exposure of resistor 108 to the range oftemperatures will be selectively controlled. In one embodiment, apermanent change in the value of resistor 108 occurs when resistor 108is exposed to a temperature between 450° C. to 850° C. for a length oftime between 1 us and 1 ms. The purpose of the exposure is topermanently change the resistance of resistor 108, without rendering itfully or partially inoperable as a circuit element, as explained in moredetail with respect to FIG. 19.

In FIG. 5, photoresist 210 of FIG. 4 has been removed, and a thin filmthermal separator 214 has been deposited adjacent to resistor 108. Thinfilm thermal separator 214 has properties of both a thermal conductorand an electrical insulator. While thin film thermal separator 214 willusually be deposited using a thin film process, in one embodiment thinfilm thermal separator 214 is an air void. In another embodiment thinfilm thermal separator 214 is Si₃N₄ having a thermal conductivity ofapproximately 20 W/m/K. In another embodiment thin film thermalseparator 214 comprises silicon carbide, Si_(x)C_(1-x) having a thermalconductivity of approximately 150 W/m/K. Silicon carbide has theproperty of being hard, very stable and resistant to cracking over widetemperature ranges. The layer 214 is selected to be one that is anelectrical insulator and has acceptable thermal properties, namely, itis highly stable and resistant to cracking over a wide range oftemperatures, also, it has a thermal coefficient of expansion in a rangeto match the surrounding layers so it will not cause stress on theresistor 108 or heater 106 as the temperature changes. Si₃N₄ and SiC aretwo acceptable materials. Additionally, thin film thermal separator 214has a high breakdown voltage, allowing it to withstand a high voltagepotential between resistor 108 and the heater 106 to be described incorrespondence with FIG. 7.

FIG. 6 shows the use of photoresist 216, a temporary layer, withintrimming system 100 to create via 211 a and via 211 d in thin filmthermal separator 214, layer 208, and insulator 204. The vias extend toand expose metal layer 206 a and 206 d. By exposing metal layer 206 aand 206 d, the next conductive layer will be electrically coupled tometal layer 206 a and 206 d while still electrically insulated fromresistor 108 by means of intervening layers thin film thermal separator214, layer 208, and insulator 204.

FIG. 7 shows a deposition of a layer to form heater 106 of FIG. 1.Heater layer 106 corresponds to heater 106 of FIG. 1. In one embodiment,heater 106 is deposited over thin film thermal separator 214 via a thinfilm process. Heater 106 is connected to metal layer 206 a and 206 d.Heater 106 has heat generating properties determined by its composition,length, width, and depth. In one embodiment, heater 106 is a thin filmresistor. In another embodiment, heater 106 comprises TaSiN having asheet resistance of approximately 100 ohms/sq. Heater 106 comprises amaterial that can withstand high intensity heat without structuraldamage. Since some heat transferred from heater 106 to resistor 108 willbe dissipated and lost in thin film thermal separator 214, heater 106needs to be able to exceed beyond 850° C. to bring resistor 108 to thattemperature. Finally, heater 106 may contain the following properties:low temperature coefficient of resistance, low noise, high linearityresistance, and integration density for radio frequency devices.

FIG. 8 through FIG. 11 show the process of connecting another metallayer to heater 106. FIG. 8 shows dielectric 220 disposed adjacentheater 106. FIG. 9 shows via openings to metal layer 206 a and 206 dthrough use of a pattern in photoresist 221. FIG. 10 shows plug 222 aand 222 b inserted into the via openings of FIG. 9. FIG. 10 also showsthe deposition of metal layer 224 a and 224 b after a photoresist hasbeen patterned, etched, and removed. Metal layer 224 a is deposited ontoplug 222 a and metal layer 224 b onto plug 222 b. The physicalconnection of metal layer 224 to plug 222 to metal layer 206 provides alow resistance path for current to flow to heater 106. As shown, heater106 shares an electrical connection with plug 222 to metal layer 206.FIG. 11 shows openings in upper layer 226. In one embodiment metal layer224 is the top-most metal layer and upper layer 226 is a passivationlayer. In this embodiment metal layer 224 a may be connected to groundand metal layer 224 b may be connected to receive current flow I_(h). Itis to be understood, however, that heater 106 is bi-directional and isnot to be limited to only having current flow in the directionillustrated in FIG. 11. In another embodiment, upper layer 226 isanother inter-metal layer insulator or dielectric upon with anothermetal layer may be deposited.

The distance between heater 106 and resistor 108 serves severalfunctions. First, the distance acts to provide electrical isolationbetween the electrical elements. Electrical isolation between theelements allows one or both of heater 106 and resistor 108 to beindependently used as a loading resistor or other circuit element suchas a thermal sensor. Second, the distance controls the heat transfercharacteristics of the trimming system. The smaller the distance betweenheater 106 and resistor 108, the more easily heat is transferred betweenthe devices. Lastly, two conductors separated by a dielectric form acapacitive element. In FIG. 11, heater 106 forms a first plate, resistor108 forms a second plate and thermal separator 214 forms a dielectricbetween the two conductors. The capacitance of the combination of thesethree elements is governed generally by the equation:

C=k*A/d,

where

C=the capacitance, the ability of conductors separated by a dielectricto hold a charge;

k=the dielectric constant;

A=the area of overlap between the conductors; and

d=the distance between the conductors.

Thus, the distance between heater 106 and resistor 108 also serves thepurpose of inversely controlling the capacitance between the twoelements. In one embodiment, connections to heater 106 and resistor 108are configured to use the two conductive layers and the thin filmthermal separator 214 as a capacitor.

FIG. 12 shows a partial top view of trimming system 100. As shown,heater 106 extends over and along the length of resistor 108. Viaconnections 211 a through 211 d illustrate electrical connectionsbetween heater 106 and metal layer 206 a and 206 d as well asconnections between resistor 108 and metal layer 206 b and 206 c. Here,heater 106 is also shown disposed on top of resistor 108. Variations onthis orientation may be made that are equivalent, these include: heater106 being shorter than resistor 108, heater 106 being the same width asor wider than resistor 108, heater 106 being disposed beneath resistor108, heater 106 being disposed to a side of resistor 108, or heater 106overlapping only portions of resistor 108.

FIG. 13 is an example of one embodiment of a top view of FIG. 12 showingthe orientation of heater 106 and resistor 108. FIG. 13 shows heater 106crossing back and forth over resistor 108 in a serpentine shape. Viaconnections 213 a and 213 d illustrate electrical connections betweenheater 106 and metal layer 206 a and 206 d, and via connections 211 band 211 c illustrate connections between resistor 108 and metal layer206 b and 206 c. The duration or number of exposures of resistor 108 toheat will vary depending upon the shape of the heater 106. Other shapesmay be implemented that are equivalent to perform the function ofheating resistor 108 with heater 106 to a temperature sufficient topermanently vary the resistance of resistor 108.

FIG. 14 shows interdigitated layout 280 illustrating an embodiment ofthe current invention. Resistive network 280 includes resistor 108,resistor 109, heater 106, heater 107, and dummy resistor cells. Dummyresistor cells are placed on either end of the interdigitated resistorsto assist with better resistor matching. They can be used as test sitesto test the actual value of the resistor before trimming so that thetrimming will have an accurate starting point. They will haveappropriate connection lines so that their resistance can be measuredfrom terminals or contact pads outside of the die or in a packaged chip.In this embodiment only one cell of each of resistor 108 and resistor109 is overlapped by a heater. This embodiment has the effect of an evenmore precise fine tuning because three-fourths of resistors 108 and 109will be largely unaffected by the application of I_(heater1) to heater106 or I_(heater2) to heater 107. In another embodiment, more or less ofa single cell of resistors 108 and 109 is overlapped by heaters 106 and107 to affect the precision of trimming the resistor. In yet anotherembodiment, more cells of resistor 108 or resistor 109 are overlapped byheater 106 or heater 107 to increase the influence of each heater on thesheet resistance, and therefore overall resistance, of the resistors.Generally, resistor fabrication recipes can be considered an estimate orcourse trim of the final values of resistors 108 and 109, and usingheaters 106 and 107 to change the resistance of resistors 108 and 109can be considered a fine or precision trim of the final values of theresistors.

In one embodiment, resistor 108 is a type of resistor which increasesits resistance value when heated, and resistor 109 is a type of resistorwhich decreases its resistance value when heated. Accordingly, a user ormaker can use a fuse to connect either resistor 108 or resistor 109 intothe circuit to increase the resistance by utilizing heater 106 or todecrease the resistance by utilizing heater 107. FIG. 19 illustrates howtwo resistors, each having an initial sheet resistance of 1 k ohm/sq,may react to the 450° C. to 850° C. temperature range, in accordancewith an embodiment of the invention. In one embodiment resistor 108 andresistor 109 are connected in series and are used to represent a singleresistor, such as Rsense or R1 of FIG. 17. In one embodiment, resistor108 and resistor 109 are connected in parallel and used to represent asingle resistor, such as Rsense or R1 of FIG. 17. In these embodiments,no fuse is used, rather both resistor 108 and resistor 109 are in series(or parallel) to provide the target resistance. Each is made of adifferent material and has one or more segments. If it is desired toincrease the resistance, then resistor 108 is heated and if it isdesired to decrease the resistance, then resistor 109 is heated.

FIG. 15 is a depiction of trimming system 100 as it may be coupled to anearby device. FIG. 15 shows trimming system 100 coupled to a transistor285. Transistor 285 includes a source 228 a, a drain 228 b, a contact230, an oxide layer 232, and a gate 234. The transistor 285 maycorrespond to the switch 104 or it may be one transistor in the circuit,such as with an op amp, as shown in FIG. 15.

FIG. 15 exemplifies how resistor 108 (now on top of heater 106), mightbe connected to additional devices. Metal layer 224 a couples plug 222 awith plug 222 c. Plug 222 c is adjacent to metal layer 206 e. Metallayer 206 e is adjacent to contact 230 b which attaches to drain 228 b.Oxide layer 232 is under gate 234 and is above drain 228 b and source228 a. The gate is also connected to metal with a contact and via plug,but these elements are not shown in this figure. Source 228 a isconnected to contact 230 a. Contact 230 a is adjacent to metal layer 206f which is adjacent to plug 222 d. Plug 222 d is also adjacentlyconnected to metal layer 224 c. Metal layer 224 c may be connected toanother device or circuit through a lateral extension of metal layer 224c, as shown in 224 a, or it may be electrically connected bywire-bonding or a series of additional plugs and metal layers. As willbe appreciated by one of ordinary skill in the art, transistor 285formed with gate 234 is just an example of a device that may be coupledto resistor 108, hence this example is not to be interpreted as the onlyconfiguration available.

FIG. 16 shows an amplifier circuit 300 using a trimmable resistor inaccordance with an embodiment of this invention. Amplifier circuit 300exemplifies the proportional influence a trimmable resistor may have onthe gain of a circuit. Amplifier circuit 300 includes an input signalVin, a resistor 304, an operational amplifier (“op amp”) 306, an outputvoltage Vout, a trimmable resistor 310, a heater 312, circuitry 316, atransistor 318, and a heater control signal Vhcs.

As has been described previously, heater 312 selectively receives acurrent I from transistor 318 which is controlled by heater controlsignal Vhcs. In one embodiment heater control signal Vhcs is a singlepulse. In another embodiment, heater control signal Vhcs is a series ofpulses that can be applied from an outside terminal or controlled by acomputer program. The computer program is stored in a computer-readablemedium such as a disk, a memory, or the like. Circuitry 316 representsadditional circuit elements that may be placed between transistor 318and heater 312. In one embodiment, circuitry 316 is merely a straightline conductor. In another embodiment circuitry 312 includes additionaltransistors for current control or voltage regulation. Heater 312selectively increases in temperature in response to current I_(heat)flowing from transistor 318. The heat from heater 312 is transferredthrough a dielectric, like air or SiO₂, to permanently change theresistance of trimmable resistor 310.

Trimmable resistor 310, also labeled Rfb, is part of an invertingamplifier configuration. The output of op amp 306 is fed back to theinverting input of op amp 306. The feedback configuration of Rfbproportionally affects output voltage Vout as follows:

Vout=−Vin*Rfb/R1.

Thus, changes in Rfb or variations in Rfb from its designed valueproportionally affect the gain of the circuit. The ability topermanently modify the value of Rfb at any time after the fabricationprocess so that the resistance aligns more closely with the desiredvalue can greatly tighten tolerances and improve performance of circuitimplementations.

FIG. 17A exemplifies additional uses of a trimmable resistor inaccordance with an embodiment of the present invention as used inintegrated circuit (“IC”) system 400. IC system 400 shows an IC senseamp 402 connected to provide an output signal to an IC microprocessor404. IC system 400 also includes circuitry external to IC sense amp 402and IC microprocessor 404, such as a current source 406, a shuntresistor Rsense, a load, a voltage regulator Vreg, a decouplingcapacitor C1, gain resistors R1 and R2, and a connection to ground.

IC sense amp 402 receives an input at Vp (voltage plus) and Vm (voltageminus) terminals and, in this configuration, produces an output on theOut terminal of IC sense amp 402. Current source 406 forces a currentthrough shunt resistor Rsense to the load. As a result of the currentflowing from current source 406, electric potential Vsense developsacross the terminals of shunt resistor Rsense. Electric potential Vsenseis passed through resistors Rg1 and Rg2 to the corresponding + and −inputs of op amp 410. The difference in electrical potential acrossinputs + and − of op amp 410 is proportionally increased by the gain ofop amp 410 and transferred to the base of transistor 411. Assuming Rg1,Rg2, and Rg3 are equal, a voltage proportional to Vsense will betransmitted via transistor 411 and resistor Rg3 to Vin, the +input of opamp 412.

Op Amp 412 is configured to be a non-inverting amplifier. Therelationship between Out, Vin, R1, and R2 is:

Out=Vin*1+R2/R1=Vin*(R2+R1)/R1.

Thus, the Out terminal of IC sense amp 402 is proportional to Vin aswell as to the sum of R2 and R1. It should be noted that when R1 issignificantly larger than R2 then the non-inverting amplifierconfiguration of op amp 412 resembles a follower (Out˜=Vin).

In one embodiment, Rsense is a thin film trimmable resistor which has aresistance that can be increased or decreased in accordance with anembodiment of the present invention.

In another embodiment, one or more of the resistors described in ICsystem 400 are implemented with thin film trimmable resistors which haveresistances that can be selectively increased or decreased in accordancewith an embodiment of the present invention.

In another embodiment, IC sense amp 402 is a high sense amp which can beused as a high voltage capacitance filter to make it robust in anelectromagnetic environment. It can also be used to provide OEMprotection because high voltage capacitances have ultra low density.

FIG. 17B illustrates IC system 400 having the features of FIG. 17A inaddition to having Rsense, R1, and R2 internal to integrated circuit402. Previously, a precision resistor such as Rsense had to be anexternal resistor because resistor tolerances on integrated circuitstend to vary widely. Since it was very difficult to manufactureprecision resistances to an exact value within tight tolerances inintegrated circuits, until this invention, Rsense was required to be anexternal resistor.

Rsense had to be an external resistance for two reasons. First a userhad to choose the needed value within a tolerance, and second, the userhad to choose a course value that would interact appropriately with alarge or small load. However, in accordance with this invention we havethe advantage of being able to have Rsense be part of the integratedcircuit. Rsense no longer has to be a component external to theintegrated circuit requiring a user to purchase and assemble additionalcomponents. The user now has the ability to select the value of Rsenseto achieve both a higher or lower resistance as well as trim it to aprecise value of the resistance. Accordingly, what used to be a timeconsuming and expensive process of purchasing the correct resistor andconnecting it into a circuit design is no longer necessary. The keyresistor, Rsense is now included in silicon on the integrated circuit.The user can now just program the value of Rsense based on the desiredend use. In one embodiment each of Rsense, R1, and R2 are all integratedinto the same integrated circuit die and are trimmable in accordancewith an embodiment of the present invention. Any of the resistances,including any of the internal resistances Rg1, Rg2, and Rg3 can betrimmable using the techniques of this invention. Accordingly, the userwill now have the ability to customize the circuit by choosing theprecise desired resistances in accordance with the desired end use, thussaving considerable money, time, and having a better performing andhigher quality end product.

FIGS. 18A-18C illustrate different steps that may occur in methods fortrimming a resistor 500, in accordance with an embodiment of the presentinvention. This method will be discussed in the context of trimmingsystem 100 of FIG. 1. However, it is to be understood that the stepsdisclosed herein may be varied in accordance with other embodiments ofthe invention.

Each of the methods of trimming described herein, including those shownin FIGS. 18A-18C, can be carried out at various stages during themanufacturing process. In a first embodiment, they are carried out atthe wafer test stage as performed by the manufacturer. In alternativeembodiments, the wafer trimming steps can be carried out after the waferhas been diced and the individual dies are being tested duringpackaging. The invention has particular advantages when used in apackage chip. For high precision circuits, it has been found thatpackaging sometimes induces parameter shift and precision loss. Inaddition, packaging can have effects on various components in thecircuit, slightly modifying the performance of different transistors,amplifier circuits, and different structures, once the chip is fullypackaged. This may be for various reasons, including the conditionswhich the die encounters during packaging, and also because of theelectrical connection changes which take place when the die is placedinside a package, ball bonded to a lead frame, and then connected toterminals outside the package. It has been found that in some examplecircuits having standard amplifiers, audio amplifiers, high speedcircuits, and current sensing, that the packaging steps providesufficient modification to the circuit that resistor trimming issometimes best performed after the packaging has been completed.Accordingly, according to one embodiment, the resistor trimming stepsshown in FIGS. 18A-18C are carried out after the packaging has beencompleted.

As described herein, all of the acts comprising the method may beorchestrated by a manufacturing processor or controller based at leastin part on execution of computer-readable instructions stored on a diskor in memory. In other embodiments, a hardware implementation of all orsome of the acts of the manufacturing method may be used.

FIG. 18A is a flowchart illustrating an embodiment of a method oftrimming a resistor. Generally, the embodiment of FIG. 18A relates to amethod of trimming a resistor having a known value while a moredesirable end value of the resistor is sought. One seeking to implementthis method would typically understand the characteristics of theresistor being trimmed. That is, to change the resistance of theresistor by some desired number of ohms or ohms/sq, the temperature andduration of the temperature of resistor exposure should already beknown. In one embodiment, a chart, table, database, or the like is usedto determine how long resistor 108 must be exposed to a certaintemperature to effect the desired change. Hence, FIG. 18A illustrates asystem without feedback while trimming a resistor where the currentvalue is known, a new value is desired, and a temperature and durationare chosen to cause the value of the resistance to change from thecurrent value to the new desired value.

In step 505, the resistance of resistor 108 is determined. This can beperformed by many methods, including: directly measuring resistor 108with a voltmeter; measuring a circuit containing resistor 108 andcalculating the resistance based upon the values of interrelated circuitelements; or measuring a similarly composed material deposited in awafer scribe line, deposited in a corner of an individual die, or thelike. In one embodiment, a test structure made of exactly the same layeras the resistor is made at a location on the wafer that can be easilyprobed. For example, if the resistor to be trimmed is made inpolysilicon, a relatively large strip of polysilicon located in thescribe line or at a testable location can be formed at the same time,using the same process steps as the resistors in the circuit. The actualresistor itself cannot be tested, but the sheet resistance of thecorresponding structure in the scribe line can be tested and the exactvalue of the resistance, as formed can thus be known. The amount ofchange needed in the resistance to achieve the target performance cantherefore be known and the value of the resistor can be changed by thisamount.

In step 510, switch 104 is selectively pulsed to cause a pulse or seriesof pulses of current to flow through heater 106 which is adjacent toresistor 108. Current flows through switch 104 because it is coupled toa voltage source V_(h). To protect heater 106 as well as the dielectricwhich is separating heater 106 from resistor 108, the current applied toheater 106 will need to be short enough to prevent overheating. Theduration of the pulse will depend upon the amplitude of the voltageapplied and the resistance of heater 106; however, application of thevoltage will resemble a current or voltage pulse. The pulse may take theform of a square wave pulse, a triangle wave pulse, a sinusoidal wavepulse, or the like. The pulse needs to bring heater 106 to a temperaturehigh enough to place resistor 108 in the range of 450° C. and 850° C.

In step 515, heater 106 reaches the desired temperature to causetemperature of resistor 108 to enter the range of 450° C. and 850° C. Inone embodiment, a chart, table, database, or the like is used todetermine how long resistor 108 must be exposed to a certain temperatureto effect the desired change. In one embodiment the resistance ofresistor 108 permanently decreases in value as the temperature ofresistor 108 increases, as depicted by TFR#1 (thin film heater) of FIG.19. In another embodiment the resistance of resistor 108 permanentlyincreases in value with temperature increase, as depicted by TFR#2 ofFIG. 19. It is noteworthy that the resistance of TRF#2 of FIG. 19increases by approximately 85% and the resistance of TFR#1 decreases byapproximately 25% within the range of 450° C. and 850° C. In oneembodiment the initial sheet resistance of the resistors represented byTFR#1 and TFR#2 is 1 k ohm/sq. Other materials or longer resistors canbe used to cause more changes in the resistance value in order toaccommodate a larger range, such as a doubling or tripling of theresistance, or cutting it by half or one-third.

Step 520 shows selectively disconnecting heater 106 from the voltagesource after resistor 108 has reached the approximate desiredtemperature. This method utilizes predetermined temperature and durationdata to produce a desired change in the resistance of resistor 108 in asingle progression through steps 505, 510, 515, and 520.

FIG. 18B is a flowchart illustrating another embodiment of a method oftrimming a resistor. In contrast to the method of FIG. 18A, FIG. 18Billustrates an embodiment of an iterative method of trimming a resistor.Rather than exposing resistor 108 to a particular temperature for apredetermined duration, this embodiment illustrates incrementallyapproaching a desired final resistance value. This embodiment will,depending upon the increments in resistance per iteration, produce amuch more accurate result that the embodiment of FIG. 18A. The cost forthe more precise result is the time consumed by iterating betweenseveral of the steps discussed below.

In step 505, the resistance of resistor 108 is determined. As discussed,this can be performed by many methods, including: directly measuringresistor 108 with a voltmeter; measuring a circuit containing resistor108 and calculating the resistance based upon the values of interrelatedcircuit elements; or measuring a similarly composed material depositedin a wafer scribe line, deposited in a corner of an individual die, orthe like.

In step 510, switch 104 is pulsed a single time to cause a pulse ofcurrent to flow through heater 106 which is adjacent to resistor 108.Current flows through switch 104 because it is coupled to a voltagesource V_(h). The duration of the pulse will depend upon the amplitudeof the voltage applied, the resistance of heater 106, and the desiredresolution of incremental changes in resistance. The finer theresolution of the incremental changes, the closer the final value ofresistor 108 will be to the desired final value of resistor 108.

In step 517, it is determined whether resistor 108 has reached thedesired value of resistance. If the desired value has not yet beenreached, then the method would iteratively return to step 505 todetermine the resistance of resistor 108, continue to step 510 to pulseswitch 104, and return to step 517. If the desired value of resistor 108had been reached, the method continues to step 520. Additionally, if thedesired value of resistor 108 had not been reached, but the resolutionof increments is course enough that an additional iteration would resultin surpassing the desired value of resistance, then this too wouldresult in progressing to step 520.

Step 520 shows selectively disconnecting heater 106 from the voltagesource by discontinuing pulses to switch 104. This step is performedafter resistor 108 has reached the desired temperature and resistancevalue. From here one would restart the process of trimming again only ifthe circuit, in which the resistor is incorporated, required additionaltuning.

FIG. 18C is a flowchart illustrating another embodiment of a method oftrimming a resistor. This embodiment includes utilizing the output ofintegrated circuit 101, into which resistor 108 is incorporated, todetermine how much a resistance needs to be adjusted. Unlike theembodiments of both FIGS. 18A and 18B, the initial value of resistor108, in isolation, is unknown. In many cases access to individualresistors is not available; however access to circuitry output, such asthe gain of an amplifier, likely will be available. In such a scenario,the embodiment of FIG. 18C, a performance based trimming method, becomesvery useful.

In step 522, circuit 101 is tested to measure its performance. In oneembodiment, circuit 101 includes a low-pass or high-pass filter andtesting the circuit includes measuring the cutoff frequency. In oneembodiment, testing circuit performance includes testing a sense ampwith a known current. In another embodiment, circuit 101 includes anamplifier configuration similar to that of FIG. 16. For example, if Rfbis initially 4.2 k ohms and R1 is 1 k ohm, the gain (Vout/Vin) will be−4.2. If however, a gain of −5 is desired the content of step 524 isperformed to produce the desired result.

In step 524, the value of resistor 108 that would produce the desiredresult is calculated. Continuing the example of the configuration ofFIG. 16, where resistor 108 is Rfb of 4.2 k, one would understand thatthe value of Rfb would need to be increased to 5 k ohms to get a desiredgain of −5. The user would need access to heater 106 to accomplish theprecision trimming of the circuit, but the user would not be required tohave direct access to the actual resistor being trimmed, whether tomeasure or modify. A voltage is provided to selected pins of the packagethat correspond to the heater. It is not necessary to blow a fuse orprovide access to the resistor to trim it, rather, the trimming can bedone with standard voltages and connecting normally available packagepins to the outside and the resistor can read the desired value usingelectrical programming on the finished product. This is clearly anadvantage over the prior art.

Similarly, the embodiment of FIG. 17B also illustrates the immenseutility in trimmable resistors within an IC. Specifically, the gain ofop amp 412 is controlled by R1 and R2. A user can measure the output ofop amp 412 at Vout and make determinations based on the differencebetween the output measured and the output expected. The user candetermine which resistor R1 or R2 to change and how much to change it toproduce the designed gain. Additionally, using internal trimmableresistors R1 and R2 saves: space by drastically reducing the size of theoverall system, the time consumed in connecting external resistors tothe system, and the cost of purchasing additional resistors to add tothe system.

Lastly, step 526 includes applying a voltage to heater 106 to cause theprevious value of resistor 108, R1, or R2 to change to the calculatedvalue of the resistor, thereby tuning the circuit 101 to perform closerto the target performance. The resistor can have its value changed instep 526 using either the method steps of FIG. 18A or 18B, or otheracceptable method.

In one embodiment, the resistors of FIGS. 16 and 17 are implemented astwo trimmable resistors in series, where each resistor has its ownindependent heater. Furthermore, as described with respect to FIG. 14,one of the resistors could have a resistance that increases when heatedto a particular temperature and the other resistor in the series couldhave a resistance that decreases when heated to a particulartemperature. Thus, a user would have the option to either increase ordecrease the resistance of a resistor comprised of two segments inseries which are composed of different resistive materials. It is to beunderstood that while FIGS. 18A through 18C describe differentembodiments of methods by which to test and trim resistor 108, variousother methods exist that are not explicitly disclosed here but thatadhere to the spirit of these embodiments.

The foregoing detailed description has set forth various embodiments ofthe devices and/or processes via the use of block diagrams, schematics,and examples. Insofar as such block diagrams, schematics, and examplescontain one or more functions and/or operations, it will be understoodby those skilled in the art that each function and/or operation withinsuch block diagrams, flowcharts, or examples can be implemented,individually and/or collectively, by a wide range of hardware, software,firmware, or virtually any combination thereof. In one embodiment, thepresent subject matter may be implemented via Application SpecificIntegrated Circuits (ASICs). However, those skilled in the art willrecognize that the embodiments disclosed herein, in whole or in part,can be equivalently implemented in standard integrated circuits, as oneor more programs executed by one or more processors, as one or moreprograms executed by one or more controllers (e.g., microcontrollers),as firmware, or as virtually any combination thereof, and that designingthe circuitry and/or writing the code for the software and or firmwarewould be well within the skill of one of ordinary skill in the art inlight of this disclosure.

When logic is implemented as software and stored in memory, it would beequivalent that logic or information can be stored on any computerreadable storage medium for use by or in connection with anyprocessor-related system or method. In the context of this document, amemory is a computer readable storage medium that is an electronic,magnetic, optical, or other physical device or means that contains orstores a computer and/or processor program and/or data or information.Logic and/or the information can be embodied in any computer readablestorage medium for use by or in connection with an instruction executionsystem, apparatus, or device, such as a computer-based system,processor-containing system, or other system that can fetch theinstructions from the instruction execution system, apparatus, or deviceand execute the instructions associated with logic and/or information.

The various embodiments described above can be combined to providefurther embodiments. From the foregoing it will be appreciated that,although specific embodiments have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the teachings. Accordingly, the claims are notlimited by the disclosed embodiments.

1. A method of forming a trimmable resistance structure in an integratedcircuit die, the method comprising: forming a resistive structure over asemiconductor substrate, the resistive structure being composed of amaterial whose resistance is permanently changeable by application ofheat thereto; forming a dielectric layer over the semiconductorsubstrate adjacent the resistive structure; forming a heater adjacentthe dielectric layer, the dielectric layer being positioned between theheater layer and the resistive structure; and forming a switch in thesemiconductor substrate configured to selectively couple the heater to avoltage supply to cause the heater to heat the resistive structure tocause a permanent change in the resistance of the resistive structure.2. The method of claim 1 wherein the resistive structure, the heater,and the dielectric material are thin films.
 3. The method of claim 1comprising forming the dielectric material on the heater.
 4. The methodof claim 1 comprising forming the resistive structure on the dielectricmaterial.
 5. The method of claim 1 wherein the dielectric includesSi₃N₄.
 6. The method of claim 5 wherein the resistive structure includesCrSi.
 7. A method comprising: forming a transistor in a semiconductorsubstrate; forming a heater layer on the semiconductor substrate;forming a dielectric layer on the heater layer; and forming a resistoron the heater layer, the transistor being configured to pass a currentthrough the heater to heat the resistor to permanently adjust aresistance of the resistor.
 8. The method of claim 7 wherein theresistive structure, the heater, and the dielectric material are thinfilms.
 9. The method of claim 7 wherein the resistor is CrSi.
 10. Themethod of claim 7 wherein the dielectric layer is Si₃N₄.
 11. The methodof claim 7 wherein heating the resistive structure decreases theresistance of the resistive structure.
 12. A method, comprising: formingan amplifier in the integrated circuit die; forming a trimmable thinfilm resistor in the integrated circuit die electrically coupled to theamplifier, a gain of the amplifier being based in part on a resistanceof the trimable thin film resistor; forming a thin film heater in theintegrated circuit die thermally coupled to the trimmable thin filmresistor; forming a transistor in the integrated circuit dieelectrically coupled to the thin film heater and configured toselectively pass a current through the thin film heater to increase atemperature of the trimable thin film resistor to permanently alter theresistance of the trimable thin film resistor; and forming a thin filmthermal separator disposed between the trimmable thin film resistor andthe thin film heater.
 13. The method of claim 12 wherein the thin filmheater is a thin film resistor.
 14. The method of claim 13 wherein theresistance of the thin film heater is less than the resistance of thetrimmable thin film resistor.
 15. The method of claim 12 wherein theresistance of the trimmable thin film resistor permanently changes withexposure to a temperature within a range of temperatures.
 16. The methodof claim 15 wherein the range of temperatures includes 450° C. to 850°C.
 17. The method of claim 12 wherein the thin film thermal separator isa dielectric.
 18. The method of claim 17 wherein the thin film thermalseparator includes Si₃N₄.
 19. The method of claim 12 wherein thetrimmable thin film resistor includes CrSi.